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Voltage drop across wire

  1. Jan 26, 2008 #1
    Hi all,

    I'd like to ask for your help in solving a problem. I'm currently working on building a small ground robot for a certain research problem. The robot is powered by batteries, and consumes a continuous 80A @ 12V when all of its systems are active. The batteries provide capacity for about 2 hrs of runtime, but since extended testing sessions are common, we would like to use a high amperage battery charger that can remain connected to the batteries indefinitely, which will continuously top off and keep the batteries charged so the robot can also run indefinitely.

    Now the question is, we can approach this one of two ways: 1. Have the battery charger on-board the robot, and run a 120VAC extension cord from the power source to the charger. This obviously presents no problems, is certainly feasible, except that now we have an extra charger on the robot which is only dead weight. So, is it possible to 2. Have the battery charger external, and run a 12V line out to the batteries on board the robot?

    The problem is, say we are talking about a 50 ft. length piece of wire, maybe 8AWG. With 80A running through this, will there be an appreciable voltage drop at the other end? I found this calculator, http://www.powerstream.com/Wire_Size.htm which says there will be a 5.1V drop, which is obviously unacceptable. However, I have no clue what calculations were performed to arrive at this answer. Can anyone help out? Thanks.

    I know my question is kind of long and convoluted. If you need any clarifications on the problem, please ask. Thanks again.
  2. jcsd
  3. Jan 26, 2008 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Look up a table of resistance for different gauges, like this one:


    Use whatever gauge is necessary to achieve the resistance you need to obtain an acceptable voltage drop.

    Alternatively, use remote sensing (run two small sense wires alongside the power cables, connected to the power cables at the robot's battery, and have the battery charger regulate its output voltage according to the sense wires).

    - Warren
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